Frank Loop Formation in Irradiated Metals in Response to Applied and Internal Stresses

The Frank loop and dislocation microstructures developed in three face-centered cubic alloys during fast reactor irradiation have been examined to determine the influence of applied and internally generated stress on loop evolution. It is shown that anisotropic stresses generate a corresponding anisotropy of Frank loop populations on the four close-packed planes. The loop populations thus represent a microstructural record of the irradiation creep processes in action. The ease of interpreting this record depends on the relative magnitudes of external and internal stresses.

Metals with low irradiation creep rates which also undergo concurrent and substantial phase changes during irradiation are subject to large and indeterminate levels of internally generated stress which render the microstructural record uninterpretable with respect to the applied stress state. When the internally generated stresses are small in comparison to the externally applied stresses, a clear record of the stress-induced preferential absorption of interstitials (SIPA) growth mechanism of irradiation creep is imprinted at low neutron fluences in the density and sizes of loops present on each set of close-packed planes. This record fades at higher fluences when the continued anisotropic formation, growth, and unfaulting of Frank loops generate a corresponding anisotropy in the resultant free dislocation network, a process which alters the competition of sinks for point defects.